Vickers Hardness
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Vickers hardness
General information
The Vickers hardness testing method (named after the English company Vickers) was developed in 1925 by Smith and Sandland. The decisive factor was that the applicability of Brinell hardness was limited due to the flattening of the steel ball, and carbide balls were not yet available at that time.
Measurement principle of Vickers hardness
The Vickers method is very similar to the Brinell method. Instead of a ball, a regular, four-sided diamond pyramid with a square base and an angle of 136° between the opposite areas is used as the indenter. The indenter is pressed vertically into the test specimen or sample with a test load F, and the impression is normally measured under a light microscope in the manner illustrated in the image after the load is removed:
| Fig.: | Schematic illustration of Vickers hardness testing with measurement technology (a), indentation creation (b), and indentation measurement (c) |
The test load must be adapted to the respective geometric and morphological conditions; as a rule, tests are performed with loads ≤ 5 N. Normally, the test load F in the Vickers hardness test is selected so that the indentation diagonal d is relatively large compared to the structural components (internal averaging) and can thus be measured more accurately (lower relative error). However, the hardness of individual structural components can also be measured specifically, in which case the geometric conditions must be selected in exactly the opposite way.
Definition of Vickers hardness
The lengths of the two indentation diagonals are determined as the measured variables. From this, the average indentation diagonal d is calculated, which is used to calculate the Vickers hardness HV.
with
| HV | Vickers hardness in N mm-2 | |
| F | test load in N | |
| A | Indentation surface in mm2 | |
| d | Average value of indentation diagonal in mm |
Comment:
In the hardness testing of plastics, the physically exact unit of measurement N mm-2 is always specified for the hardness value. When testing the hardness of metallic materials, the test force F is multiplied by 0.102 (≈1/9.80665 = 1/gn) (gn – acceleration due to gravity). The resulting value has the dimension kp. However, since the kp was removed from the hardness designation, this unit is no longer specified; only the hardness value without dimension is given. The kp can still be found as a specification for the test force levels, especially on older hardness testing machines.
When performing the Vickers test, the resulting indentation is normally measured after unloading, but there are also ways to determine it under load. In this case, the test specimen surface and thus the indentation diagonal are observed through the diamond indenter, which allows, for example, real-time conclusions to be drawn about creep behaviour.
While the Vickers method is standardized for metallic materials and therefore test load recommendations exist for the micro, small load, and macro ranges, the Vickers method is not standardized for plastics.
Instrumented hardness testing with Vickers indenter
In instrumented hardness testing with Vickers indenters, the entire indentation process is recorded during loading and load–indentation depth diagrams are recorded. Due to the pyramid geometry, the indentation depth h is related to the indentation diagonal d by the relationship h = d/7.0006.
In addition to the hardness value, instrumented hardness testing can be used to determine other mechanical material parameters such as the modulus of elasticity (indentation modulus), work hardening exponents, and viscoelastic properties.
See also
- Conventional hardness testing
- Ball indentation hardness
- Scratch hardness
- TABOR relationship
- Instrumented scratch testing
References
- Blumenauer, Horst (Ed.): Werkstoffprüfung. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1994) 6th Edition, (ISBN 978-3-342-00547-6; see AMK-Library under M 3)
- Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2022) 3rd Edition, p. 180 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56990-807-5; see AMK-Library under A 22)